Nullizygous mice display neonatal lethality, thin-walled atria, and vascular abnormalities including abnormal branchial arch artery development, cardiac outflow tract anomalies, and reduced vascular smooth muscle around some vessels. Homozygotes for a knock-out allele show prenatal lethality, small fetal size, subcutaneous hemorrhage, atelectasis, impaired cardiac and pulmonary vascular development, and enhanced vascular endothelial cell migration and vessel formation.